I would make no concession on rent. Maybe share the cost of any products needed to install the common area garden. KEEP IN MIND-If your turning an area that needs zero maintenance into an area that needs constant maintenance-regardless if the tenants good intentions , this area will always need maintenance in the future. What happens if tenant loses interest? They move out? What will happen to this space?
I used to be on a facilities planning board for a museum. They had a donor that wanted to pay to have a perennial garden installed by a professional landscape company. The proposed drawing was beautiful. Despite the good intentions and my warning of future costs to maintain to the committee, it was passed and the install happened. The first year it was beautiful and the grounds company didn't have to do much to maintain it. The following year the contract to take care of the grounds received an addendum increased of $1200 to maintain this bed. They museum didn't want to pay (they couldn't really) and decided to rely on some volunteer members that wanted the bed to stay. A year after that it was a mess. Overgrown, the black-eyed-Susan and purple cone flowers that didn't get deadheaded in time took over everything. A year after that they paid the landscape company to return the bed to a grass area. The bottom line is- there is a cost to beautiful landscapes and gardens not everyone knows how to consider.